It is well known that a speaker system sometimes generates an unusual crackling sound when a power supply of the audio amplifier is turned on or off or a standby mode of the audio amplifier is cancelled, or in a transition period while voltages of applicable sections of the audio amplifier circuit are rising. Such an unusual crackling sound is called a “pop sound” that is considerably annoying to a hearer and makes the hearer uncomfortable. Further, such a pop sound sometimes damages the speaker.
As a technology for reducing such a pop sound, Japanese Laid Open Patent Application No. 2004-304441 (hereinafter also referred to as a “Patent Document 1”), for example, discloses a technology to lower the generation of the pop sound by raising a reference voltage of the audio amplifier at the activation or deactivation of the audio amplifier such that the reference voltage of the audio amplifier exhibits a raised cosine waveform.
FIG. 4 is a circuit diagram illustrating an example of a such a related art pop sound reduction circuit disclosed in Patent Document 1, and FIG. 5 is a timing chart illustrating an operation example of the related art pop sound reduction circuit of FIG. 4. FIG. 5 illustrates waveforms of respective voltages at positions A4 through C4 and waveform of drain currents i401 and i402 of NMOS transistors M403 and N404 of FIG. 4.
In FIG. 4, when the power supply of the audio amplifier is turned on to supply a power voltage Vdd to the pop sound reduction circuit, a capacitor C401 is charged via a resistor R401 such that the voltage at A4 rises in a logarithmic curve. Since the voltage at A4 is supplied to respective gates of an NMOS transistor M401 and a PMOS transistor M402, the drain current i401 is changed based on the voltage at A4.
That is, if the voltage at A4 is low, the PMOS transistor M402 is in an on-state and the NMOS transistor M401 is in an off-state. Thus, little drain current i401 is flown.
When the voltage at A4 rises to turn on the NMOS transistor M401, the drain current i401 has begun to flow, and the amount of the drain current i401 increases with the increase of the voltage at A4. The combined resistance of the NMOS transistor M401 and PMOS transistor M402 is reduced to its minimum while the drain current i401 is increased to its maximum when the voltage at A4 is at around ½ of the voltage Vdd of the power supply voltage.
When the voltage at A4 increases further, on-resistance of the NMOS transistor M401 is further lowered. However, on-resistance of the PMOS transistor M402 is lowered further than that of the NMOS transistor M401. Accordingly, the drain current i401 has begun to decrease.
When the voltage at A4 is further increased approximately to the same level as the power supply voltage Vdd, the PMOS transistor M402 is turned off. Thus, no drain current i401 is flown.
At this moment, the drain current i401 is flown as the drain current of the NMOS transistor M403. Since the NMOS transistors M403 and M404 form a current mirror circuit, the drain current i402 of the NMOS transistor M404 is also flown as in the same manner as the drain current i401.
A capacitor C402 is charged with the drain current i402 so that the voltage of the capacitor C402 is raised. Note that if an audio reproduction circuit 401 outputs a constant output voltage (e.g., ground voltage), the voltage output at B4 is raised due to a change similar to a terminal voltage of the capacitor C402. This is because there is a proportional relationship between an output voltage of an operating amplifier 402 and the terminal voltage of the capacitor C402.
That is, the voltage at B4 has a voltage waveform that gradually rises at the beginning of the wave form, where the voltage at B4 is obtained immediately after the power supply is turned on, rapidly rises at a mid point of the voltage waveform, and gradually rises again toward an end point of the voltage waveform. Accordingly, a peak value of the voltage waveform at C4 may be suppressed, thereby lowering the pop sound.
In addition, Japanese Laid Open Patent Application No. 2005-109654 (hereinafter also referred to as a “Patent Document 2”) discloses another technology for lowering the generation of the pop sound. In this technology, plural current source supplies are provided and the capacitor C402 of FIG. 4 is charged by switching the plural current source supplies to generate the voltage having the voltage waveform at B4 illustrated in FIG. 5.
However, in a case of the related art pop sound reduction circuit in FIG. 4, the voltage at A4 rises in a logarithmic curve. Accordingly, the voltage rapidly rises immediately after the power supply is turned on, but its rising rate gradually slows down with time. As a result, on-state duration of the PMOS transistor M402 shortens while on-state duration of the NMOS transistor M401 increases. Accordingly, an asymmetric waveform of the drain current is obtained unlike a symmetric waveform of the drain current i401 illustrated in FIG. 5.
Further, since a value of the drain current i401 varies with respective threshold voltages of the NMOS transistor M401 and the PMOS transistor M402, the drain current i401 is significantly varied based on manufacturing conditions of the transistors.
Moreover, with this configuration, the NMOS transistor M403 and the NMOS transistor M404 form a current mirror circuit, and the capacitor C402 is connected between a source and a ground voltage Vss of the NMOS transistor M404. Accordingly, the drain current of the NMOS transistor M404 is reduced as the voltage of the capacitor C402 rises. As a result, an accurate mirroring effect may not be obtained.
This indicates that the drain current i402 is significantly reduced and becomes significantly smaller than the drain current i401 with time. As a result, the symmetry of the drain current i402 illustrated in FIG. 5 is more significantly changed than that of the drain current i401.
Thus, a voltage change at B4 is gradual toward the end of the waveform, so that a longer time is required to set the reference voltage. In addition, frequent generation of harmonics in the voltage at B4 inhibits the effect of lowering the pop sound.
Further, if the plural current sources are provided to charge the capacitor corresponding to the capacitor C402 of FIG. 4, the malfunction of the pop sound reduction circuit of FIG. 4 may be fixed. However, in this case, the plural current sources to charge the capacitor are switched with time, and hence the harmonics are frequently generated in the voltage at the switching of the plural current sources.
Accordingly, in the related art pop sound reduction technologies, the voltage at an output end (at B) of a differential amplifier circuit gradually changes toward the end point of the voltage waveform generated after the power supply is turned on, so that a longer time is required to set the reference voltage and frequent generation of harmonics in the voltage at B suppresses the effect of lowering the pop sound.